1. Field of the Invention
The present invention relates to duplex phase stainless steels exhibiting superplasticity and to a hot working method thereof.
2. Prior Art
Generally, duplex phase stainless steels are utilized after being subjected to a solution treatment in the last production step thereof in which they are quenched after being heated to a temperature of from about 1000.degree. to 1100.degree. C. After the treatment, they exhibit two phases: .alpha. (ferrite) and .gamma. (austenite). The duplex stainless steels thus obtained are known to have superior strength, toughness, and weldability, as well as corrosion resistance. Thus, the demand therefor is increasing in various fields. Duplex steels, however, are difficult to work because of their duplex phase structure, and this difficulty in working has greatly limited their fields of application.
Research to develop processes for producing in greater quantities duplex stainless steels with the above-mentioned advantages have led to the adoption of a method of decreasing the content of impurities such as sulfur and oxygen which have harmful effects on hot working processes. This approach has enabled duplex stainless steels to be worked into articles of simple shapes such as pipes and plates or to be forged into articles of relatively simple shapes. The production of articles of more complicated shapes such as pipe joints and valves by hot working, however, is still extremely difficult. Thus, such articles of complicated shapes could only be manufactured by machining or casting processes which are low in efficiency and yield.
Recent rapid developements in the field of superplastic working processes have found application to duplex stainless steels as promising methods of working them into complicated shapes. Duplex stainless steels are already reported to exhibit certain degrees of superplasticity ranging from 400 to 500% [see, for example, G. I. Smith, B. Norgate and N. Redley, Met. Sci., 10 (1976), p. 182 et seq.]. The duplex stainless steels thus far known, however, show some superplasticity only under conditions of extremely low strain rate (from 10.sup.-4 to 10.sup.-5 s.sup.-1, for example). Thus, it has not been possible to efficiently apply superplastic working to duplex stainless steels under practical conditions for hot working.